Field of the Invention
This invention relates generally to the recovery of valuable metal anions, specifically oxyanions containing molybdenum, tungsten, vanadium, or uranium, from aqueous solutions such as leach liquors containing such oxyanions and also containing contaminant anions such as PO.sub.4.sup.-3, SO.sub.4.sup.-2, NO.sub.3.sup.-, Cl.sup.-, ClO.sub.3.sup.-, and ClO.sub.4.sup.-. More specifically, the invention is an electrolytic process using ion-permeable membranes to facilitate the recovery of valuable metal oxyanions, and of compounds formed from such oxyanions, in purer form by lowering the contaminant anion content and cation content of aqueous solutions containing the valuable metal oxyanions and the contaminant anions. Furthermore, the invention permits recovery of the contaminant anions in usable form.
Valuable metals such as molybdenum, tungsten, vanadium, and uranium, are often recovered through processes which include caustic leaching of concentrates, to produce streams containing the desired metal as a valuable metal anion. The leaching operation also dissolves contaminant anions, such as PO.sub.4.sup.-3, SO.sub.4.sup.-2, NO.sub.3.sup.-, Cl.sup.-, ClO.sub.3.sup.- and/or ClO.sub.4.sup.-. For instance, in one process the tungsten-containing ore wolframite is digested with caustic to form a solution of sodium tungstate which contains other dissolved ions such as phosphate. Scheelite (calcium tungstate) is known to be treated by digestion with soda ash (or sodium carbonate), also to form a sodium tungstate solution contaminated with e.g. phosphate ion. In addition, acid leaching of tungsten ores will produce a tungstate product contaminated with the anion of the acid used, such as chloride anion.
The anions carried along with the valuable metal oxyanions are considered contaminants because they can reduce the concentration of the valuable metal anion and because they can cause the formation of useless by-products when the leach solution is subsequently treated to recover the desired metal. In addition, the contaminant anions can pose operational problems by limiting or interfering with the processes used to separate the valuable metal anions from the leach liquors. Many conventional separatory techniques produce waste streams of contaminant anions, which must somehow be disposed of. It is also desirable to increase the ratio of valuable metal oxyanions to cations in the solution, so as to facilitate subsequent recovery of the desired metal values.
Thus, it is desirable to separate the contaminant anions from the valuable metal anions in leach liquors, and preferably to do so while recovering the cations and contaminant anions in the form of useful byproducts, without introducing additional chemical species into the system.
The present discovery for achieving this objective lies in the field of electrolysis, which is concerned with processes in which ion-permeable permselective membranes are interposed between the anode and cathode of an electrolytic cell to control the passage of dissolved ions toward the electrodes.
Briefly, a permselective membrane may be described as a three-dimensional network of an insoluble organic polymer. Bound into the chains of the network are water insoluble reactive chemical groups which are free to dissociate and are capable of exchanging only cations or only anions, in roughly the same manner as granular ion exchange resins. The interstices between the chains are filled with water and are of such a size that ions can pass through only by displacing ions already on the reactive groups lining the passages. Since this displacement occurs by an ion exchange mechanism, and since the reactive groups of a given membrane will exchange only ions of a given sign, the membrane will readily pass ions of that given sign while resisting the passage of ions of the opposite sign. The membranes are formed so as to be substantially water impermeable, that is, they do not permmit water to pass in significant quantities. Permselective membranes thus possess many characteristics of granular ion exchange resins; however, since the principal mechanism by which the membranes operate is ion transfer through the membrane, the membranes do not require separate chemical regeneration or stripping steps for recovery of the desired molecular species as do granular exchange resins. Instead, continuous elution occurs on the electrode side of the membrane and continuous loading occurs on the feed side of the membrane. This feature means that processes utilizing permselective membranes are suitable for continuous operation.
Surprisingly, it has been discovered that under certain conditions an anion-permselective membrane will favor passage of the contaminant anions from a contaminated solution which also contains valuable metal oxyanions, following which the valuable metal oxyanions can be passed through the same or another anion-permeable membrane to permit their recovery, for instance as ammonium compounds, in the anolyte.